Objective To investigate the effect of FK506 on the in vitro ostengenic potential of rat bone marrow-derived mesenchymal stem cells(MSCs)and the dose-response effect of FK506(5-5000 nmol/ L)on the ostengenic potential of MSCs in vitro.Methods MSCs derived from primary culture were sub- cultured for 16 days under four conditions:(1)?-MEM containing L-ascorbicacid-2-phosphate(AsAP)and?-glycemphosphate(?-GP)(basic culture medium)as a control;(2) AsAP and?-GP(basic culture medi- um) plus dexamethasone;(3)AsAP and?-GP(basic culture medium)plus FK506,(4)AsAP and?-GP (basic culture medium)plus FK506 and dexamethasone.Osteogenie potential was determined by testing os- teoblastic morphology,cell proliferation,alkaline phosphatase(APase)activity,bone nodule formation and the expression of osteocalcin mRNA.Results FK506 promoted the proliferation of MSCs,induced mineral- izing bone-like nodule formation,and increased APase activity and the expression of osteocalcin mRNA.The data also showed that the efficacy of FK506 was greater when used in combination with dexamethasone.Opti- mal ostengenesis was achieved with?-MEM containing 0.25mmol/L AsAP,10mmol/L?-GP,10nmol/L dexa- methasone and 50nmol/L FK506.Conclusion FK506 has powerful ostengenic ablility.It can be consid- ered as an osteogenic agent to repair bone defects.